Audio valve amplifiers for musical instruments, in particular for electric guitars, comprise a valve power amplifier stage connectable to the input of an acoustic speaker and comprising at least one thermionic valve, e.g. a thermionic valve in a single-ended configuration or two thermionic valves in a push-pull configuration. The thermionic valves used may be triodes, tetrodes or pentodes. The valve power amplifier stage comprises a polarization circuit for polarizing the anode, cathode and control grid of one or more thermionic valves. The polarization of thermionic valves determines the working point of the thermionic valves and the maximum output power deliverable by the thermionic valves.
These audio amplifiers are normally used under high distortion conditions of the valve power amplifier stage in order to obtain specific sound features. This means that the power amplifier stage should often work at the maximum output power and that musicians should tolerate considerable acoustic pressure levels.
In order to reduce the acoustic pressure of the distorted sound, a solution is known of switching the operating mode of the thermionic valve, e.g. from pentode to triode, or changing the operating class of the thermionic valves, e.g. moving from class AB to class A. In both cases, a reduced number (up to three) of discrete maximum output power levels is obtained.
Another known solution is that of inserting an adjustable resistive power attenuator between the output of the power amplifier stage and the input of the acoustic speaker, by means of which the acoustic pressure generated by the acoustic speaker can be adjusted among a higher number of discrete levels. However, the resistive power attenuator appreciably alters some features of the audio valve amplifier, such as for example damping and the impedance curve seen from the amplifier. Furthermore, the thermionic valve always works at maximum power also when the acoustic pressure produced by the acoustic speaker is set to low levels.